1. Field of the Invention
The present invention relates to a toner for use in an image forming process utilizing, for example, electrophotography, electrostatic recording or electrostatic printing and using a toner recycling system; and to a developer, a toner container, an image forming apparatus (developing apparatus) and a process cartridge using the toner.
2. Description of the Related Art
In electrophotographic image formation, a latent image is electrostatically formed by charging and exposing on an image bearing member having a photoconductive layer of a photoconductive substance, the latent electrostatic image is developed using colored particles of toner to thereby form a visible toner image. The toner image is transferred to a transfer material such as a sheet of paper and is fixed thereon by action of, for example, heat, pressure or solvent evaporation to form an image output (copied image). Among various image fixing methods to fix a toner image known in the art, a heat roller fixing method is widely employed for high thermal efficiency and for image fixing at high speed. Basically, (1) a toner for use in the heat roller fixing method must be reliably fixed at low temperatures, that is, must have excellent image fixing properties at low temperatures, and (2) it must be resistant to migrating to the heat roller when it is fused in image fixing, that is, it must have excellent hot offset resistance. In addition, to form a sharp copied image, the toner must be stably present as a powder without aggregation during use or while being stored, that is, it must have excellent storage stability. To stably form good images without fogging in repetitive cycles, the toner must be resistant to crash or damage due to mechanical impact or pressure in a developing device.
Recently, more image forming apparatuses for developing a latent image formed on a photoconductor using a toner include a cleaner for removing residual toner on a photoconductor drum after transferring and a recycling device for recycling the toner removed by the cleaner to a developing device japanese Patent Application Laid-Open (JP-A) No. 60-41079). However, when a toner used in such an image forming apparatus using a recycling system may often invite a decreased image density, toner deposition on the background of images, fogging, and attachment of carrier particles with an increasing number of repetitive cycles of image formation. This is because the toner is deformed and broken by action of shearing force applied in a recycling process to have a decreased charge ability, and finely divided particles of the toner formed as a result of breaking reduces the charge imparting ability of the carrier. In monochrome copying systems which mainly employ an oil-less toner, a wax used as a releasing agent bleeds out from the surface of the toner particles by action of heat in an image forming apparatus or toner stress in cleaning or toner recycling system and adheres to the carrier to thereby invite a decreased charge.
Certain toners comprising a crosslinked polyester resin as a binder resin are used as a toner for use in a recycling system (JP-A No. 59-14144, No. 58-14147, No. 60-176049, No. 60-176054, No. 62-127748, and No. 62-127749). However, when these toners frequently receive mechanical external force due to, for example, stirring in a developing device as a result of recycling, they may be often broken to form fine powder, and the fine powder contaminates the carrier particles to decrease the charge ability of the carrier particles to thereby yield toner particles having an insufficient charge, and these toner particles then contaminate a developer carrying member or other components to decrease image developing properties.
To recycle toners satisfactorily, various investigations and improvements have been made on toners, image forming methods and image forming apparatus. For example, an attempt has been made to reuse a residual toner on an image bearing member after an image is transferred to an image-receiving member in an electrostatic image forming process. Such a residual toner has been stocked in, for example, a toner reclaim bottle, and the bottle has been discarded as an industrial waste. Such a discarded toner pollutes the environment and is a waste of resources. To avoid discarding of toners and to use all the toners, investigations have been made on toner recycling systems.
For example, JP-A No. 63-246780 discloses a technique in which a conveying path for conveying a recycled toner from a cleaner to a developing device is provided and the recycled toner is used as part of a toner feeding to the developing device. JP-A No. 01-118774 discloses a technique in which a cleaner is not used and a residual toner after transferring is recovered by a developing device. JP-A No. 06-51672 discloses a technique in which a chargeable rotating member is provided which recovers a toner by action of electrostatic force from a region of an image bearing member used for toner transfer and which releases the toner onto another region of the image bearing member not used for toner transfer.
However, these techniques each have defects and are not satisfactory. The technique disclosed in JP-A No. 63-246780 requires the conveying path of the toner such as a pipe and toner conveying means such as a screw or belt and thus invites a large-sized complicated apparatus. In the technique disclosed in JP-A No. 01-118774, a residual toner once adhered to an image bearing member cannot be significantly recovered in the developing device, thus adheres and attaches to the image bearing member, often resulting in toner deposition on the background of images or stained color portions due to insufficient exposure. This technique is not ready for abnormal circumstances such as paper jamming and may adversely affect subsequent processes after the toner adheres to the image bearing member. Various reports have been made in addition to the above descriptions, but are not satisfactory.
Based on strong demands for higher image quality, development of electrophotographic apparatus suitable for forming high-quality images and toner developers for use in the apparatus has increased. A toner for forming high-quality images preferably comprises spherical toner particles having a small particle diameter and a sharp (narrow) particle diameter distribution. Such toner particles having a sharp particle diameter distribution and a spherical shape behave in an identical manner in development to thereby significantly improve fine dot reproducibility.
However, such a conventional toner having a small particle diameter and a sharp particle distribution is not suitable for use in a recycling system. The toner cannot be sufficiently cleaned in the recycling system. In particular, it cannot be stably cleaned in blade cleaning. Under these circumstances, various techniques have been proposed on toners to improve the cleaning ability. For example, a technique for converting toner particles from spherical to irregular shape has been proposed, in which the flowability of the toner is decreased due to its irregular shape, and the irregular toner particles become easily stoppable with the blade. However, if the toner particles have excessively irregular shapes, the toner particles behave unstably in development to thereby decrease the fine dot reproducibility.
By converting the toner particles to have irregular shapes, the cleaning reliability is improved but image-fixing properties are deteriorated. Namely, the irregular-shaped toner particles are packed in a lower density in a toner layer on the transfer material before image fixing, and heat conducts in the toner layer at a lower speed in image fixing to thereby deteriorate image-fixing properties at low temperatures. In particular, when an image is fixed at a lower pressure, the heat conducts at a further lower speed to thereby inhibit image-fixing at low temperatures.
In contrast, JP-A No. 11-133665 discloses a toner having a Wadell practical sphericity of 0.90 to 1.00 and comprising a polyester. However, the toner is substantially spherical and cannot solve the cleaning problems.
Objects and Advantages
Under these circumstances, an object of the present invention is to provide a toner and a developer which can be used in a toner recycling system without deformation and break-down, with less change in a surface of the toner, are free from decreased durability and quality, fogging, decreased image density, toner deposition on the background of images, and toner change with environment and can form images with good quality, as well as to provide an image forming apparatus and a detachable process cartridge using the toner.
Another object of the present invention is to provide a toner and a developer which can form images with high quality and excellent fine-dot reproducibility and can have high reliability in cleaning.
Yet another object of the present invention is to provide a toner and a developer which have excellent image-fixing properties at low temperatures.
Still another object of the present invention is to provide a toner and a developer which can satisfy all the above requirements.
A further object of the present invention is to provide a toner which has a high transfer efficiency, produces less residual toner after transfer and can form images with high quality.